A vapour compression system normally comprises a compressor, a condenser, an expansion device, e.g. in the form of an expansion valve, and an evaporator arranged in a refrigerant path. Refrigerant is circulated in the refrigerant path, and is alternatingly compressed and expanded, while heat exchange takes place in the condenser and the evaporator, thereby providing heating or cooling for a closed volume.
When a vapour compression system is started, e.g. by starting the compressor, the amount of refrigerant present in the evaporator, or the filling degree of the evaporator, is not known. In order to obtain a maximum cooling efficiency, it is desirable to reach maximum filling degree of the evaporator as quickly as possible. On the other hand, it should be ensured that liquid refrigerant is prevented from passing through the evaporator and entering the suction line, since it may damage the compressor if liquid refrigerant reaches it.
U.S. Pat. No. 5,771,703 discloses a control system for controlling the flow of refrigerant in a vapour compression system. The control system causes optimal use of the evaporator coil by ensuring that the refrigerant in the coil is in the liquid state. A temperature sensor at the evaporator coil exit senses refrigerant temperature and the control system regulates refrigerant flow so that the liquid dry out point, i.e. the transition between liquid state and superheat state, occurs in the vicinity of this sensor. Thereby the vapour compression system is operated at optimal superheat during normal operation.